Composite materials are widely used throughout industry. Composites have proven to provide superior strength and weight characteristics over traditional materials in many applications. A composite sandwich structure is often used in applications where minimizing weight is important. These composite sandwich structures are composed of solid composite face sheets with a lighter core material “sandwiched” between. Composite sandwich structures, while light, are often not strong enough to withstand the stresses inherent with certain applications such as use in joints or in bends. Traditionally, metal has been used in joints and around bends due to its superior isotropic strength characteristics. The metal joint or flange is bolted onto the composite sandwich structure using a splice plate. This procedure complicates the assembly process and increases fabrication costs.
It is desirable to use a solid laminate composite structure instead of metal in those situations where a sandwich structure is not adequate and then transition to a sandwich structure in one integral part without using bolts and splice plates. However, several problems arise when using a solid laminate in a bend and then transitioning to a sandwich structure. First, a composite's strength is planar. The geometry of a bend can induce high interlaminar tension stresses in a composite that can cause delamination and failure.
Another problem arises when transitioning from the solid laminate composite to a composite sandwich structure. The conventional method of transitioning to the sandwich structure involves terminating inner plies of the solid laminate, leaving a void between the top and bottom plies. The remaining top and bottom plies become face sheets, between which a core material is inserted. The termination of the inner plies of solid laminate creates an edge that abuts the core material of the sandwich structure. This interface creates two problems.
The first issue involves the termination of the individual plies within the solid laminate. Many composite materials are fabricated by layering individual plies at various orientation angles within a stacking sequence to produce a product with superior strength characteristics in various directions within a plane. However, it is difficult to terminate the plies with high precision due to the varied orientation angles. This results in a ragged or uneven edge at the interface between the solid laminate and sandwich core. The uneven edge leads to the development of defects at this interface when the part is processed. Additionally, the interface between the solid laminate and the core material induces high interlaminar stresses when the part is structurally loaded, which can lead to a material failure.
Therefore, in light of the above, there is need in the art for a method of fabricating an integral composite structure that includes solid laminate, strong enough to use in joints and in bends, and core sandwich material, in one integral composite part, without the need for bolts and splice plates.